1. Technical Field
The present invention generally relates to a process for preparing vinyl chloroformate and its use in making biomedical devices.
2. Description of Related Art
Vinyl chloroformate (VCF) is used as a starting raw material for the synthesis of many medical devices such as those disclosed in, for example, U.S. Pat. Nos. 5,310,779, 5,449,729, 5,610,252 and 6,166,236, the contents of which are incorporated herein by reference. For example, two of the key monomers used in the production of hydrogel contact lenses are (N-vinyloxycarbonyl)-3-aminopropyltris(trimethylsiloxysilane) (RD325) and vinylcarbamate-capped polydimethylsiloxane (RD352). The current synthesis of RD325 consists of reacting VCF with aminopropyltris(trimethylsiloxysilane). The current synthesis of RD352 involves a triflic acid catalyzed ring opening polymerization of octamethylcyclotetrasiloxane with a vinyl carbamate butyl-capped tetramethyldisiloxane (V2). V2 is prepared by the reaction of VCF with 1,3-(4-hydroxybutyl)-1,1,3,3-tetramethyldisiloxane. Both syntheses are relatively straightforward and result in a high yield of product. A significant drawback, however, of both reactions is that the monomer VCF is used.
Currently, it is believed that VCF is only available via a multi-step synthesis which involves the use of phosgene and organomercury reagents. However, organomercury reagents are undesirable in that they present heightened environmental concerns.
Accordingly, efforts have been made to avoid the use of VCF in making medical devices such as contact lenses. For example, U.S. Pat. No. 6,423,862 discloses a multi-step method for making a vinyl carbamate that does not necessitate the use of phosgene and organomercury intermediates. Another example is U.S. Pat. No. 7,402,689 which discloses methods of forming vinyl(thio)carbamates and more particularly to a vinylchloroformate-free method of synthesizing (N-vinyloxycarbonyl)-3-aminopropyltris(trimethylsiloxysilane). Yet another example is U.S. Pat. No. 7,745,564 which discloses a method of synthesizing device forming monomers using N-(vinyloxycarbonyloxy)succinimide. However, additional methods of making vinyl or allyl(thio)carbamates that do not require the use of organomercury reagents and are relatively straightforward are still needed.
Accordingly, it would be desirable to provide an improved process for making vinyl chloroformate in a simple, cost effective manner that does not involve the use of organomercury reagents.